In compliance with the principles of energy saving, consumption reduction, high efficiency and environmental protection, higher temperature, higher pressure and longer service life represent a developmental trend of devices in the fields of electric power, refining and chemicals, metallurgy, aviation, etc. For example, a single ultra-supercritical generator unit in a power plant has a power of 1000 MW or more, operating parameters of 600-650° C./32-35 MPa, and a design life of 30 years. An advanced turbine of an aero-engine has a front inlet temperature of up to 1980-2080° C., a thrust weight ratio of 15-20 or more, and a maximum service life of more than 40000 hours. The 700° C. thermal power and 4th generation nuclear power technologies under development with great efforts nowadays are also based on high temperature and pressure parameters and a long design life. With increasing promotion of the operating parameters of the devices, heat resisting materials such as ferrite steels, martensite steels and austenite steels cannot continue to meet the operating requirements of the various components. The effectuation of these process devices entails substantial use of nickel-based alloys with higher strength and better creep characteristics at high temperature.
Nickel-based high-temperature bolt, using nickel-based alloys as raw materials, is the generic term for all types of mechanical parts used to fasten and connect two or more elements as a whole at high-temperature environment. They mainly include bolts, studs, nuts, etc., widely used in the fields of energy, refining and chemicals, metallurgy, aviation, etc.
The current design methods for nickel-based high-temperature bolts are based on strength theories, and have the following general procedure: acquiring operating parameters; selecting a material; determining the pretension force; determining an arrangement and dimension of the bolts; analyzing the stress applied on the bolts; and checking the strength under various working conditions (considering the influence of relaxation).
However, fracture incidents of nickel-based high-temperature bolts occur from time to time. For example, some steam turbine bolts made of GH4145 alloy fractured in Jingyuan Second Power Co., Ltd. (2011); and a batch of bolts composed of Inconel 783 alloy cracked in several ultra-supercritical power generation units (2012); etc. Such incidents caused shutdown or production halt, leading to enormous economic loss. The main reason is that fracture toughness is specifically related to time and significantly reduces after a long time service of the nickel-based alloys at high temperature, although the nickel-based alloys has better high-temperature strength and anti-relaxation property. Hence, the critical point in design is to ensure that a nickel-based high-temperature bolt will not fracture in its service life. For this kind of materials with such properties, although, the conventional design methods for bolts based on strength theories appear to provide perfect safety factor, a decisive factor of the materials' fracture property is not taken into consideration in the design process, so the safe operation of a device cannot be guaranteed.
Therefore, the conventional design methods for bolts are not suitable for nickel-based bolts with superior strength and inferior resistance to fracture at high temperature. Thus, in this field, it is urgent to develop a new design method for resistance to fracture of nickel-based bolts at high temperature.